My U.S. Pat. No. 3,425,381, assigned to the assignee of the present application, describes a method of securing an end closure to a container body by means of a triple rolled seam including seven layers of sheet metal (convention double seams comprise five layers of sheet metal). My earlier Patent described a method of forming such a triple rolled seam and apparatus for carrying out the method.
This technique provides a method of securing the radially outwardly extending flanges of the container body and of the end closures together in such a way as to maximize strength, to provide a perfect seal and to minimize amount of sheet material used.
In the method described in my above-mentioned U.S. patent a suitably shaped seaming roll drives back the radially outwardly extending flanges inward in a continuous manner to roll the peripheral portion of the end closure flange until the free outer edge thereof faces the yet undeformed container body end flange, near the outer periphery thereof. After this initial rolling phase, the two overlying flanges are rolled together in a continuous manner until the triple rolled seam is obtained. Such a triple rolled seam comprises a tight spiral made up of seven alternating layers of the end closure and container body sheet metal.
To minimize the amount of sheet material used, the peripheral portion of the end closure flange is rolled back on itself in the initial rolling phase which produces an initial rolled section which is the heart of the seam and must be as small as possible.
The seaming rolls used in this triple seaming method have a groove with a curved profile or contour having a number of different radii of curvature.
The prior art method achieves the sought after results when the thickness of the container body and end closure sheet material is less than one millimeter. Experience has shown, however, that with container body and end closure sheet material thicknesses greater than one millimeter it is difficult to form the initial rolled portion or section having a radius as small as possible when using seaming rolls designed for sheet metal thicknesses less than 1 millimeter.
The present invention is intended to overcome the difficulties associated with triple roll seaming sheet material thicknesses greater than 1 millimeter, while still being applicable to smaller sheet thicknesses.